Buck-boost power converters can convert an input voltage into an output voltage higher than, equal to or lower than the input voltage and can generally be operated with wide input voltage range. Therefore, buck-boost power converters are widely used in power management applications. FIG. 1 illustrates a topology 50 of power switches of a conventional buck-boost power converter. The topology 50 comprises a first power switch 11, a second power switch 12, a third power switch 13 and a fourth power switch 14. The first power switch 11 and the third power switch 13 are coupled in series between an input port IN and a reference ground, and have a common connection referred to as a first switching node SW1. The second power switch 12 and the fourth power switch 14 are coupled in series between an output port OUT and the reference ground, and have a common connection referred to as a second switching node SW2. An inductor is coupled between the first switching node SW1 and the second switching node SW2. A capacitor is coupled between the output port OUT and the reference ground. An input voltage VIN at the input port IN is converted to an output voltage VOUT at the output port OUT through controlling the power switches 11-14 on and off.
If the first power switch 11 and the second power switch 12 are N channel power switching devices (e.g., N channel field effect transistors, N channel double diffused metal oxide semiconductor transistors etc.), bootstrap circuits should be provided to enhance the driving capability of the corresponding drivers. Generally, a first bootstrap capacitor CB1 coupled between the input port IN and the first switching node SW1 is configured to generate a first bootstrap voltage VBST1 referenced with the voltage at the first switching node SW1 to enhance the driving capability of corresponding driver to fully turn the first power switch 11 on and off, and a second bootstrap capacitor CB2 coupled between the output port OUT and the second switching node SW2 is configured to generate a second bootstrap voltage VBST2 referenced with the voltage at the second switching node SW2 to enhance the driving capability of corresponding driver to fully turn the second power switch 12 on and off.
The first bootstrap voltage VBST1 and the second bootstrap voltage VBST2 can be respectively generated through charging the first bootstrap capacitor CB1 and the second bootstrap capacitor CB2. However, under certain operation conditions, such as when the buck-boost power converter operates under light load or no-load condition, charges on the first bootstrap capacitor CB1 and the second bootstrap capacitor CB2 may not be enough, leading to decrease in the first bootstrap voltage VBST1 and the second bootstrap voltage VBST2 to the extent that may not be able to drive the first power switch 11 and the second power switch 12 to switch on and off normally. Therefore, when the first bootstrap voltage VBST1 and the second bootstrap voltage VBST2 are lower than a predetermined threshold, the first bootstrap voltage VBST1 and the second bootstrap voltage VBST2 should be refreshed, i.e., charging the first bootstrap capacitor CB1 and the second bootstrap capacitor CB2 so as to recover the first bootstrap voltage VBST1 and the second bootstrap voltage VBST2 to a desired bootstrap voltage value. Therefore, it is desired to have a solution for efficiently refreshing the first bootstrap voltage VBST1 and the second bootstrap voltage VBST2.